This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an applications for SINGLE MODE OPTICAL FIBER earlier filed in the Korean Industrial Property Office on Dec. 30, 1997 and there duly assigned an Ser. No. 77780/1997, and for SINGLE MODE OPTICAL FIBER earlier filed in the Korean Industrial Property Office on Nov. 4, 1998 and there duly assigned Ser. No. 47134/1998.
1. Field of the Invention
The present invention relates to a dispersion shifted optical fiber, and more particularly, to a dispersion shifted optical fiber whose refractive index varies with the radius of a core.
2. Description of the Related Art
In general, an optical fiber for long-distance, superhigh-speed, and wide-band communications must have the characteristics of low loss, low dispersion, and low dispersion slope at wavelengths in use. An optical fiber having such characteristics is usually a dispersion shifted fiber or a non-zero dispersion shifted fiber. The distribution forms of the refractive indices of these fibers are changed into various structures to satisfy the aforementioned optical characteristics.
Such a change in the refractive index can be obtained by providing an annular region to a core whose refractive index varies in the shape of a triangle or allowing the core to have a double core structure corresponding to a convex type refractive index.
In conventional core arrangements, such as that disclosed in U.S. Pat. No. 5,553,185 to Antos et al., entitled Controlled Dispersion Optical Waveguide, the difference in refractive index between the cladding and the core is made larger by depressing the reflective index of an annular region adjacent to the core, thereby obtaining a low dispersion slope.
In order the widen the difference in refractive index between the core and the cladding, the refractive index of the cores increased or that of the cladding is depressed. However, in the first method, optical loss is increased by a dopant which is used to increase the refractive index, making it impossible to have a refractive index over a predetermined level.
In the second method, loss is suddenly increased in a long wavelength region by a depressed region. In order to solve this problem, the ratio of the radius of a cladding to that of the core must be large.
U.S. Pat. No. 4,447,127 to Cohen et al., entitled Low Loss Single Mode Fiber, discloses an arrangement in which the leakage loss depending on each wavelength of an optical fiber when the ratios between the radius of a cladding to that of a core are 6 and 7.
However, when a modified chemical vapor deposition is applied to the optical fiber manufacturing method as noted above, the conventional method still has problems in that it is difficult to manufacture an optical fiber preform having a large aperture and it takes a great deal of time to manufacture the optical fiber.
The following patents each discloses features in common with the present invention: U.S. Pat. No. 5,675,688 to Nouchi et al, entitled Dispersion-Shifted Monomode Optical Fiber, U.S. Pat. No. 5,799,123 to Oyobe et al, entitled Dispersion Compensating Fiber, U.S. Pat. No. 4,755,022 to Ohashi et al., entitled Zero Dispersion Single Mode Optical Fiber with Center Core And Side Core In The 1.5 xcexcM Wavelength Region, U.S. Pat. No. 5,703,986 to Brehm et al., entitled Monomode Optical Fiber, U.S. Pat. No. 5,649,044 to Bhagavatula, entitled Dispersion Shifted Optical Waveguide, U.S. Pat. No. 4,820,018 to Melman et al., entitled Optical Fiber For Light Amplification, U.S. Pat. No. 4,852,968 to Reed, entitled Optical Fiber Comprising A Refractive Index Trench, U.S. Pat. No. 4,893,896 to Tanaka et al., entitled Energy Transmission Optical Fiber, U.S. Pat. No. 5,115,486 to Bader et al., entitled Flexible Optical Graded-Index Profile Fiber For Transmission Of Laser Radiation With High Output With Substantial Preservation Of The Mode Structure, U.S. Pat. No. 4,465,334 to Siemsen et al., entitled Multilayer Fiber Light Conductor, U.S. Pat. No. 4,664,474 to Tanaka et al., entitled Optical Fiber And Process For Producing The Same, U.S. Pat. No. 4,715,695 to Nishimura et al., entitled Fiber For Optical Transmission, U.S. Pat. No. 5,729,645 to Garito et al., entitled Graded Index Optical Fibers, and U.S. Pat. No. 4,641,917 to Glodis et al., entitled Single Mode Optical Fiber.
To solve the above problems, it is an object of the present invention to provide a dispersion shifted optical fiber whose refractive index varies to obtain low dispersion and low dispersion slope.
Accordingly, to achieve the above objective, there is provided a dispersion shifted optical fiber comprising: a first core having a constant refractive index within a predetermined radius from the center of an optical fiber; a second core which covers the first core and has a refractive index which decreases from the refractive index of the first core with an increase in its radius; and a cladding which covers the second core and has a refractive index smaller than the minimum refractive index of the second core.